1. Field of the Invention
The present invention relates to a method for rationally setting a black plate (hereinafter referred as "B plate") used in combination with cyan, magenta and yellow plates (hereinafter called as "C, M and Y plates") in a multicolor plate-making apparatus.
More specifically, the present invention relates to a novel method for not only determining a color-separation curve for a C plate or any other basic color plate, but also rationally setting a color-separation curve for a black (B) plate in association with the basic color plate, by using a specific tonal conversion formula.
2. Description of the Related Art
The importance of a black plate has been widely recognized in the field of multicolor plate-making. For example, the black plate is recognized as an important plate for finally adjusting a gradation and color-tone of a color printed picture.
As is well known, color separation work consists of to converting picture information values (density information values or physical values correlating thereto) of a color-film original picture in continuous gradation into a printed picture in half-tone gradation and a color-correction work. Particularly in recent years, more attention is paid to the importance of a tonal conversion technique from a viewpoint of faithful reproduction and color reproduction of an original picture. This is a natural result since when a delicate and fine tone (gradation) belonging to a color-film original picture is once lost in the course of production of a printed picture, the tone which has been lost cannot be reproduced even if compensated with a color-correction technique. In this field, however, color-correctionism where the color correction is regarded as the most important matter has been dominant for a long time, even now in the technique for production of printed pictures, therefore color-scanners as tools for color-separation work are, of course, affected with this thought.
Meanwhile, the tonal conversion work is carried out on the basis of a color-separation curve. This color-separation curve is a curve, referred also as "a tone reproduction curve", which is expressed in an orthogonal coordinate system of which a horizontal axis represents density values of an original picture and a vertical axis represents dot area % values. This curve is extremely important in defining tonal conversion (conversion of continuous tone into halftone).
A multicolor plate-making work is carried out on the basis of color-separation curves defined for three color plates (C, M and Y) and black (B) plate, respectively. Out of these plates, the black (B) plate is utilized in the multicolor plate making work in such a manner that:
(1) in three-color plate-making using three color plates (C, M and Y) therein, black color is not yielded because of impurities of the printing inks, namely, spectral reflectance curves of the respective colors largely deviate from ideal ones. To compensate for this, the black plate is adopted;
(2) the black plates is used in association with a technique of under color removal (UCR) in a scanner work.
The black plate is used in relation with the under color removal (UCR) for adjustment of a shadow balance (fog or density in the shadow area), maintenance of a gray balance in the middle area or the whole area, appropriate printing (speed up of printing, adjustment of time required to dry the inks), or reduction of the ink cost by reducing a part of the color plates (C, M and Y) (UCR).
Meanwhile, it is common in this field that modes of use of the black plate are classified according to a density value from which black is started to be used for each of the plates [this being called as "starting point (STP)"] or an amount of black to be used (UCR amount). The former STP and is set in a short range (e.g., 1.0 density) is referred as "skelton black", while what the highlight point of a black plate and is started from an area of low density, similar to color plates, is referred as "full black", such black plate being used as a main plate in achromatic plate-making. The latter full black is called as "gray component replacement (GCR)", or termed in Europe as "achromatic color reproduction".
A color-separation curve of the black plate is suitably altered by the scanner operator or the like, from the viewpoint described above, and further from a viewpoint of which tone, soft like or hard like, is provided to a resulting print, etc.
A problem in setting color-separation curves lies in the following point. Conventional color-separation curves for the respective color plates (C, M and Y) are set on the basis of experiences and intuition of the operator, or are set on the basis of experiences of the scanner manufacturer and stored in a memory of the scanner.
On the other hand, reflecting the above circumstances of the color-separation curves for the color plates (C, M and Y), a color-separation curve for the black plate (B) is also quite experiential, thus unsatisfactory when seen from a viewpoint of the thought that the color-separation curves of both color plates and black plate should be rationally set.
The way of thought about rational setting of color-separation curves of color and black plates is that a gradation of pixels ranging from the highlight area (H) to the shadow area (S) of an original picture is reproduced with fidelity of 1:1, in other words, picture information values of the original picture is tonal-converted so as to obtain a natural gradation when observed by human vision. The color-separation curves of the plates should be set on the basis of this thought, taking into consideration organic relationships among the plates.
However, there is a difficult factor obstructing a rational approach in setting of a color-separation curve of a black (B) plate. The factor is a constitution of a halftone dot (dot) arrangement, when seeing it microscopically.
This point will be described hereinafter in detail in the case of setting a color-separation curve for a black (B) plate under UCR or GCR.
In order to rationally set color-separation curves, a color-separation curve for black (B) plate has required to be set on the basis of gray and/or black component(s) which is/are produced by accumulation of three color printing inks, besides color-separation curves for the respective color plates of C, M and Y have to be set after under color removal. In this case, how the gray and/or black component is formed throughout a range from the H to S area of a color printed picture by accumulation of the three printing inks of C, M and Y has not been substantiated by a rational theory which can give an explanation totally to the whole areas. Relating to this point, difficulties existing in build-up of the theory would be easily understood because micro-structures of halftone dots respectively set to pixels on a printed picture, namely sizes of dots set to the H, middle and S areas, and their constitutional arrangements differ from each other. This point should be, however, overcome anyhow for the purpose of improvement of the printing techniques.
The present invention is to approach a rational setting of a color-separation curve for a black (B) plate, on the basis of color-separation curves for color plates (C, M and Y) (C becoming a basic plate in common practise), as will be described later.
The present inventors have honestly studied in order to make the technique for setting color-separation curves (tonal conversion curves, tone reproduction curves) rational, which technique forms a crux of the color-separation work. In the course of the investigation, the present inventors found that it is possible to produce a printed picture having excellent tonal characteristics from any type of original picture, regardless of its picture quality (for example, an original which is extremely difficult to be reproduced, such as under- or over-exposed, high- or low-key, variously color-fogged, etc.) in the following manner. That is, instead of the conventional technique for setting a color-separation curve for, for example, cyan (C) plate defined on the basis of a relationship between density information values obtained via a red (R) filter, which is in a relationship of a complementary color with cyan (C) and is used for producing the C plate, and dot area % values, adopted in the present invention are photographic characteristic curves of a color-film photographic material, on which an original picture has been photographed, to convert the density information values into light exposure values. And then these obtained light exposure values are then converted by conducting a specific tonal conversion formula to determine dot area % values, or to set a color-separation curve defined on the basis of a relationship between light intensity values and dot area % values, so that a printed picture having a superior quality may be produced (Japanese Patent Application Nos. 1-135825 and 1-212118)
Incidentally, it should be noted that, in the present invention, the meaning of the term of "light exposure value" is included in a conception of light intensity value, as will be described later, hence hereinafter will be referred as "light intensity value".
Conventional color-separation curves for the color plates, which are defined on the basis of the relationship between density information values and dot area % values, will be hereinafter referred as "D axis color separation curves" since picture information values obtained from a density axis of a characteristic curve of a photographic material. To the contrary, color-separation curves for the color plates, defined on the basis of light intensity values, which values become an object in the present invention, and the dot area % values will be hereinafter referred as "X axis color-separation curves" since the picture information values obtained from a light intensity axis or the horizontal axis (X axis) of the characteristic curve of the photographic material. It should be noted that the latter X axis color-separation curve is clearly distinguished from the former D axis color-separation curve in this invention.